Synthetic aperture radar (SAR) generates high-resolution images without having to use a large antenna that may be required by conventional “real aperture” radar systems. SAR systems image a body by generating a plurality of radar pulses configured to strike the body being imaged and to be reflected back to SAR imaging equipment. The plurality of pulses are transmitted along a range between the SAR imaging equipment to the body being imaged as the SAR imaging equipment moves relative to the body being imaged in a cross-range or azimuth direction that is perpendicular to the range or in a direction with a forward or a backward squint angle from the range direction. Because of the relative movement between the SAR imaging equipment and the body being imaged, the relative displacement between the SAR imaging equipment and the body acts as a synthetic aperture capable of high resolution imaging of the body. Imaging data is collected from data reflected from the pulses. Extensive processing is involved in integrating the imaging data to generate radar images.
Using SAR systems presents a number of challenges. One of the challenges is correcting phase errors in the data returned by the plurality of pulses. Phase error may result, for example, from inaccurate correction for relative motion between the SAR imaging equipment and the body being imaged, turbulent atmospheric conditions, or poor calibration of the imaging equipment. The phase error causes the imaging data to be, in effect, smeared in the cross-range or azimuth direction. Autofocus techniques have been developed to estimate the phase error and correct the imaging data to focus the radar images. Calibration of the SAR imaging equipment may degrade over time; thus, autofocus techniques may be particularly valuable in correcting phase error in SAR imaging equipment disposed on orbiting satellites or interplanetary probes where recalibrating the equipment may be difficult or practically impossible.
Imaging data collected by SAR systems are subject to gain error as well as phase error. Radar image quality generally is more sensitive to phase error than to gain error. Nonetheless, residual gain error resulting from poor hardware calibration or other causes may undermine the integrity of imaging data collected using SAR systems and, thus, reduce the quality of resulting radar images.